FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional nonmagnetic spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional nonmagnetic spacer layer 16 is conductive. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel with the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high.
To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Current can be driven in one of two configurations, current in plane (“CIP”) and current perpendicular to the plane (“CPP”). In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 10/10′ (up or down as seen in FIG. 1A or 1B). Typically, in memory applications, such as magnetic random access memory (MRAM) applications, the conventional magnetic elements 10 and 10′ are used in the CPP configuration.
FIG. 2 depicts another conventional magnetic element 50 utilizing layers having perpendicular magnetization. The conventional magnetic element 50 is a spin tunneling junction. The magnetic element 50 may be used in a memory cell. The magnetic element 50 includes a conventional pinned layer 60 having a magnetization 62, a barrier layer 70, and a conventional free layer 80 having a magnetization 82. The conventional pinned layer 60 and conventional free layer 80 are ferromagnetic and have their magnetizations 62 and 82, respectively, perpendicular to the plane of the layers 60 and 80, respectively. As used herein, perpendicular describes the direction normal to the plane of the layers of a magnetic element.
The conventional free layer 80 may includes a high spin polarization layer such as Co (not separately depicted in FIG. 2), and a rare earth-transition metal alloy layer such as GdFeCo (not separately depicted in FIG. 2). The conventional pinned layer 60 includes a high spin polarization layer such as Co (not separately depicted in FIG. 2), and a rare earth-transition metal alloy layer such as TbFeCo (not separately depicted in FIG. 2).
In order to overcome some of the issues associated with magnetic memories having a higher density of memory cells, it has been proposed that spin transfer be utilized for the magnetic elements 10 and 10′. In particular, spin transfer may be used to switch the magnetizations 19/19′ of the conventional free layers 18/18′. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic element 10. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), and F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, the conventional free layer 18′ should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10/10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10/10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18′ of the conventional spin tunneling junction 10′. For example, the magnetization 19′ of the conventional free layer 18′ can be switched from a direction antiparallel to the magnetization of the conventional pinned layer 14′ to a direction parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). Thus, the majority electrons traveling from the conventional pinned layer 14′ have their spins polarized in the same direction as the magnetization of the conventional pinned layer 14′. These electrons may transfer a sufficient portion of their angular momentum to the conventional free layer 18′ to switch the magnetization 19′ of the conventional free layer 18′ to be parallel to that of the conventional pinned layer 14′. Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′. When current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction), majority electrons have their spins polarized in the direction of magnetization of the conventional free layer 18′. These majority electrons are transmitted by the conventional pinned layer 14′. The minority electrons are reflected from the conventional pinned layer 14′, return to the conventional free layer 18′ and may transfer a sufficient amount of their angular momentum to switch the magnetization 19′ of the free layer 18′ antiparallel to that of the conventional pinned layer 14′.
Although spin transfer functions, one of ordinary skill in the art will readily recognize that it may be relatively difficult to write to the conventional magnetic elements 10 and 10′ using spin transfer. In particular, a relatively high current density of greater than approximately 107 A/cm2 is generally required for spin transfer to become observable and useful in switching the magnetization 19 or 19′ of the free layer 18 and 18′, respectively. The high current density is typically achieved by utilizing a conventional magnetic element 10 or 10′ having small, sub-micron lateral dimensions together with a high bias current. For example, for a magnetic element having lateral dimensions on the order of 0.1 μm by 0.2 μm, a bias current of about two milli-amperes is typically used. A reduction in the current density required to switch the magnetization 19 or 19′ of the free layer 18 and 18′, respectively, is desired for a variety of reasons. For magnetic memory applications, such as MRAM, a low power consumption and small isolation transistor dimensions are desirable. A high current density consumes greater power. In addition, the isolation transistor is typically coupled to each magnetic element. A high current density through the magnetic element requires a larger saturation current for the isolation transistor. The saturation current is proportional to the transistor size. Thus, the size of the transistor and the memory cell are increased by a high current density. Smaller memory cells and higher density memories are desirable. Such goals would be facilitated by the use of smaller transistor. Thus, the goals of low power consumption and high memory density are difficult to achieve when using a high current density. In addition, for the magnetic element 10′, the insulating barrier layer 16′ may undergo dielectric breakdown due to the presence of high current densities. Thus, the reliability of the magnetic element 10′ is adversely affected.
A prevailing spin transfer model is given in the publication: J. C. Slonczewski, “Current-Driven Excitation of Magnetic Multilayer,” Journal of Magnetism and Magnetic Materials, vol. 159, p 159, L1–L5 (1996). For the magnetic elements 10 and 10′, the film plane is along the x-y plane. The z-direction is directed upwards (perpendicular to film plane) in FIG. 1B. The critical switching current density Jc is the current required to switch the magnetization direction of a free layer for a given lateral dimensions. The switching current density for the magnetization 19 and 19′ of the free layers 18 and 18′, respectively, is described by:Jc∝αMst(Heff//+2πMs)/g  (1)Where                α represents the phenomenological Gilbert damping;        t is the thickness of the free layer 18 or 18′;        Ms is the saturation magnetization of the free layer 18 or 18′;        Heff// is the in-plane effective field;        g corresponds to an efficiency of spin transfer switching; and        2πMs is due to the demagnetization field perpendicular to the film plane.        
The in-plane effective field includes the in-plane uniaxial field, an external magnetic field, dipolar fields, and exchange fields. The demagnetization term, 2πMs, is on the order of thousands of Oe and dominates over the in-plane effective field term, which is on the order of hundreds of Oe. Thus, although conventional magnetic elements can use spin transfer as a switching mechanism, the switching current is high due to the large value of 2πMs term. For the reasons discussed above, a high switching current is undesirable for magnetic memory application. Accordingly, what is needed is a system and method for providing a magnetic memory element that can be more easily switched using spin transfer at a low switching current, while providing a high output signal. The present invention addresses the need for such a magnetic memory element.